From pieces of bark from the shell you can do the little boat. For the best stability will stick to the bottom of the boat a piece of plasticine. In this clay to stick the mast (match) with a sail.
Try and catamaran. To do this, just connect the two ships.
Sails can be made of simple paper and paint, and it is possible that they don't get wet from polyethylene.
Let boats in the Creek and in the pelvis, in the bath in a puddle.